add mutex in bitmap

db/slotpage.h

@@ -132,6 +132,7 @@ public:
   }
 
   void dealloc_slot(size_t slot_num) {
+    mtx.lock();
     const size_t byte = slot2byte(slot_num);
     const size_t off  = slot2offset(slot_num);
     char *target_byte = get_bitmap_byte(byte);
@@ -139,9 +140,11 @@ public:
     RESETBIT(target_byte, off);
 //    set_bitmap_byte(byte, target_byte);
     first_empty_slot = first_empty_slot < slot_num ? first_empty_slot:slot_num;
+    mtx.unlock();
   }
 
   size_t alloc_slot() {
+    mtx.lock();
     size_t target_slot = first_empty_slot;
     char *start_byte = get_bitmap_byte(slot2byte(first_empty_slot));
     const size_t off = slot2offset(first_empty_slot);
@@ -166,6 +169,7 @@ public:
         first_empty_slot = byte2slot(i) + 1;
       }
     }
+    mtx.unlock();
     return target_slot;
   }
 
@@ -195,6 +199,7 @@ private:
   std::vector<char *> bitmaps_;
   size_t size;
   size_t first_empty_slot;
+  std::mutex mtx;
 };